This invention relates to a process for the production of a polyurethane backing or air frothed foam on a substrate comprising the steps of a) mixing a polyisocyanate and a polyol blend in the presence of a catalyst to form a reaction mixture, b) applying the reaction mixture to a substrate, and c) curing the reaction mixture to form a polyurethane backing or air frothed foam on the substrate. This process requires specific catalyst compositions which are alkyl substituted organotin compounds containing sulfur bridging groups.
Many polyurethane applications use catalysts which are heat activated so that mixtures of the co-reactants have long potlives at room temperature, but react almost immediately when heated to a temperature above the catalyst activation temperature. Current state-of-the-art delayed action catalysts include mercury catalysts, bismuth catalysts, and amine catalysts blocked with carboxylic acids. Each of these groups of catalysts has disadvantages associated with their use. Mercury based catalysts are poisonous and, thus, serious health risks are associated with their use. Catalysts based on bismuth are water sensitive and deactivate in the presence of moisture. Accordingly, these catalysts are not suitable for any application in which water is present or added. The acid-blocked amine catalysts have an unpleasant odor associated with their use, especially when the polyurethane mixtures are cured in an oven at temperatures above 250.degree. F. This unpleasant odor also remains in the final product, making these catalysts unsuitable for some applications.
Other known delayed action catalysts include various tin-sulfur catalysts such as, for example, tin mercaptoacetates, tin mercaptides and tin sulfides. Of these, the tin mercaptoacetates are known to be sensitive to both acids and to water, which limits their usefulness to application areas which are free from acids and water. The tin mercaptides show some delayed action, but the activation temperature is too low to be of much use commercially. Both the tin mercaptoacetates and the tin mercaptides are more reactive at room temperature than is typically desirable for delayed action catalysts. Finally, the tin-sulfides have an unpleasant odor associated with their use. The unpleasant odor of these catalysts make these unpopular delayed action catalysts also.
U.S. Pat. No. 3,813,424 discloses a process for the manufacture of dialkytin oxide. This process comprises reacting alkyl iodide directly with metallic tin to form dialkytin iodide, followed by hydrolysis to the form corresponding dialkyltin oxide. Dialkyltin oxides are effective urethane catalysts, but do not have the delayed action feature desired in many applications.
Light stable elastomers and a process for their production are disclosed by U.S. Pat. No. 5,714,562. These elastomers comprise the reaction product of an isocyanate prepolymer, a chain extender and a catalyst selected from i) solid delayed action catalysts having a melting point of greater than about 60.degree. C., ii) alkyl substituted organotin catalysts containing alkylmercaptide ligands which are liquid at room temperature, and iii) alkyl substituted organotin catalysts containing sulfur bridging groups which are liquid at room temperature. Light stable elastomers are based on aliphatic diisocyanates, the reactivity profile of which is quite different from the more widely used aromatic polyisocyanates. Due to the much lower reactivity of aliphatic vs. aromatic isocyanates, it is much easier to make heat activated systems based on aliphatic isocyanates.
Various organotin catalyst compositions are disclosed in U.S. Pat. No. 5,646,195. This patent describes delayed action dialkyltin sulfide catalysts that are useful in carpet backing applications. The delay in gellation due to these dialkyltin sulfide catalysts provides improved handling and backing quality.
Processes for the production of polyurethane carpet backing, various polyisocyanates and catalysts for these processes are described in U.S. Pat. Nos. 5,462,766 and 5,558,917. The process of U.S. Pat. No. 5,462,766 forms a polyurethane by the reaction of an isocyanate and a polyol mixture wherein the isocyanate is based on polymethylene poly(phenylisocyanate) and has a monomer content of less than 55%, a 2,2'- and 2,4'-diphenylmethane diisocyanate content of less than 3%, a functionality of less than 2.5, an isocyanate group content of 25 to 30% and a urethane group content of about 2 to 6%. Suitable catalysts for this process include organometallic catalysts and tertiary amines, particularly those which are heat activated. Organonickel catalysts were used in the examples of U.S. Pat. No. 5,558,917.
These organonickel catalysts have long been known to the carpet backing industry. U.S. Pat. Nos. 3,772,224 and 3,849,156 describe their use specifically in polyurethane carpet backing formulations. However, the amount of catalysts used in these formulations is relatively high, usually from 0.5 to 3.0% by weight, based on the total weight of the polyol mixture used to form the polyurethane.
The process disclosed by U.S. Pat. No. 5,558,917 forms a polyurethane from the reaction of a polyisocyanate and a polyol wherein the polyisocyanate is characterized as having a functionality of less than about 2.4, an isocyanate group content of 25 to 30% and a urethane group content of from about 2 to 6%. This polyisocyanate is based on polymethylene poly(phenylisocyanate) and comprises from about 5 to 25% of 4,4'-diphenylmethane diisocyanate, and from about 20 to 50% of 2,2'-diphenylmethane diisocyanate. Suitable catalysts include tertiary amines, and organometallic catalysts such as, nickel acetylacetonate as in the examples.
The present invention has many advantages over the existing art. The catalysts of the present invention are used in relatively low amounts from about 0.01 to 0.5% by weight, based on the total weight of the polyol blend. The activation temperature is sufficiently high so that premature cure is reduced. The catalysts do not generate odors in the process steps or in the resulting products. Also, these catalysts maintain their activity in the presence of moisture. All of these improvements are especially advantageous in the production of carpet backing and frothed foam products.